In Q2 2006, the peak of the housing bubble, more than 99% of counties were less affordable compared to their historic levels. In Q1 2012, that figure dropped to less than 0.5% of counties.

Nationwide, during the first quarter of 2016, the average wage earner would have had to spend 30.2% of their monthly wages to pay for a median-priced home costing $199,000.

That represented an increase from the 26.4% of wages the average earner would have been required to spend in Q1 2015.

Home prices were the most affordable during the first quarter of 2012, when the average wage earner spent 22.2% of their monthly wages to pay for a home. Home prices were the least affordable in the second quarter of 2006, when the average wage earner spent 53.2% of their monthly wages to buy a home.

The top 20 county housing markets that were least affordable during the first quarter of the year compared to historic levels included counties in Denver; New York City; Omaha, Neb.; Austin, Texas; San Francisco and St. Louis.

The most affordable housing markets compared to historic levels included counties in Boston; Baltimore; Birmingham, Ala.; Providence, R.I.; and Chicago.

Housing prices could be making an impact on the overall level of homeownership. The U.S. Census Bureau reported homeownership, as of June 30, 2015, fell to 63.7%, its lowest level since 1967.

RealtyTrac's affordability index is based on the percentage of average wages a homeowner would need to make monthly house payments on a median-priced home with a 30-year fixed rate mortgage and 3% down payment. The index also takes into account property taxes and insurance.

The company used publicly recorded deed data and average wage data from the Bureau of Labor Statistics in 456 counties with a combined population of 221 million people.